Do Impedance (ohms) is important when you want to choose headphones for using them with mp3 player or computer?
If some headphones have 250 ohms it means they can't be used without amp?
Same question about max input power of headpones. - More power means that I can't use them with low end player?

Hi, sure it is..look on some hifi forum.
for 250ohms impedance,it wants already headphone amplifier.I have sound card with headphone output max. 600ohm. (but its extreme)
What sound card you have?
If you want headphone to pc on onboard sound card,buy low impedance (50ohm)

Nevermind (it's just bad)
Then after some reading I've found this
Audio-Technica ATH-M50
IMO looking fine for mp3 player and computer. Am I right?

http://www.cdprehravace.wz.cz/sluchatka/zebr.htm

Havent experience with Audio-Technica,but this can be good solution for PC and MP3 player if you dont think about headamp.But audio speech will be much depleted/degraded.if you want good (audiophile) sound you need good psu of sound (sound card or some DAC) with headamp.

http://www.head-fi.org/

My setup:
Grado SR125 headphones
HT Omega Claro Halo soundcard

Turdbeaches X90's for CoD.

Get Sennheiser headphones. Doesn't matter which, Sennheiser always has the best of the best! You can't get better sound quality from a headphone.

Take a look at http://www.fischeraudio.com/card.php?id=69
Useful for mp3 player and pc and sound quality also by reviews worth even more than they costs

As I changed title of thread I'd like to ask you guys about headamps.
Portable and home use. Anything you know.
Prices etc

I've bought Fiio E5 headamp and was good with Audio Technica's M50 but I've damaged it and I had to buy something new
So I've decided to buy Fiio E7 wich is higher model (than E5) and it also works as stand-alone sound card via USB and that's the sound I've been looking for

Now I can recommend it as very good hardware in (reasonably) low budget.